Selective laser solidification apparatus and method

ABSTRACT

Selective laser solidification apparatus is described that includes a powder bed onto which a powder layer can be deposited and a gas flow unit for passing a flow of gas over the powder bed along a predefined gas flow direction. A laser scanning unit is provided for scanning a laser beam over the powder layer to selectively solidify at least part of the powder layer to form a required pattern. The required pattern is formed from a plurality of stripes or stripe segments that are formed by advancing the laser beam along the stripe or stripe segment in a stripe formation direction. The stripe formation direction is arranged so that it always at least partially opposes the predefined gas flow direction. A corresponding method is also described.

This is a Continuation of application Ser. No. 16/587,146 filed Sep. 30,2019, which is a Continuation of Application No. 14/766,627 filed Aug.7, 2015, which is a National Stage Application of PCT/GB2014/050417filed Feb. 13, 2014, which in turn claims priority to U.S. ProvisionalApplication No. 61/774,215, filed Mar. 7, 2013. Priority is also claimedto British Application No. 1303920.1 filed Mar. 5, 2013, and BritishApplication No. 1302602.6 filed Feb. 14, 2013. The entire disclosures ofthe prior applications are hereby incorporated by reference herein intheir entirety.

The present invention relates to selective laser solidification and inparticular to an improved selective laser melting process and apparatusin which the direction of laser movement across the powder bed iscontrolled relative to the direction of gas flow.

BACKGROUND

Additive manufacturing or rapid prototyping methods for producingcomponents comprise layer-by-layer solidification of a material, such asa metal powder material, using a laser beam. A powder layer is depositedon a powder bed in a build chamber and a laser beam is scanned acrossportions of the powder layer that correspond to a cross-section of thecomponent being constructed. The laser beam melts or sinters the powderto form a solidified layer. As explained in more detail below withreference to FIG. 2 , it is typical to melt or sinter the desiredpattern in the powder layer using a series of stripes. In particular, itis known to advance so-called hatch lines back and forth along aplurality of stripes in turn to construct the desired pattern in thepowder layer. After selective solidification of a layer, the powder bedis lowered by a thickness of the newly solidified layer and a furtherlayer of powder is spread over the surface and solidified as required.

During the melting or sintering process, debris (e.g. condensate,unsolidified particles of powder etc) is produced within the buildchamber. It is known to introduce a gas flow through the build chamberin an attempt to remove debris from the chamber in the gas flow. Forexample, the M270 model of machine produced by EOS GmbH, Munich,Germany, passes a flow of gas from the top of the build chamber towardsthe powder bed and various exhaust vents collect the gas forrecirculation. The gas flow in the M270 machine is thus turbulent andhas no well defined flow direction. The newer M280 model of machineproduced by EOS comprises a series of gas outlet nozzles located to therear of the powder bed that pass a flow of gas to a series of exhaustvents that are located at the front of the powder bed. In this manner, aplanar layer of gas flow is created at the surface of the powder bed.This planar gas flow arrangement has, however, been found by the presentinventors to produce high surface roughness and non-uniformity of thesolidified metal layers generated by the melting process.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is providedselective laser solidification apparatus, comprising; a powder bed ontowhich a powder layer can be deposited, a gas flow unit for passing aflow of gas over the powder bed along a predefined gas flow direction,and a laser scanning unit for scanning a laser beam over the powderlayer to selectively solidify at least part of the powder layer to forma required pattern, the required pattern being formed from a pluralityof stripes or stripe segments, each stripe or stripe segment beingformed by advancing the laser beam along the stripe or stripe segment ina stripe formation direction, characterised in that the stripe formationdirection is always at least partially opposed to the predefined gasflow direction.

The present invention thus relates to selective laser solidificationapparatus, e.g. selective laser sintering or selective laser meltingapparatus, in which a powder layer is deposited on a powder bed. A laserscanning unit directs a laser beam onto the surface of the powder layerto solidify (e.g. melt or sinter) selected parts of the powder layer toform a required pattern (e.g. a pattern corresponding to thecross-section of a 3D object that is being constructed). This selectivesolidification is performed by dividing the area to be scanned by thelaser beam into a plurality of stripes or stripe segments. Each stripeor stripe segment is formed by advancing the laser beam along the stripeor stripe segment in a stripe formation direction. As explained below,in a preferred embodiment the laser scanning unit rapidly moves (e.g.scans or steps) a laser spot across each stripe or stripe segment toform a hatch line which is advanced along the stripe or stripe segmentin a stripe formation direction. A gas flow unit provides a flow of gas(e.g. a planar gas flow) over the powder bed whilst the stripes arebeing scanned.

The present inventors have discovered a number of problems that occurwhen using commercially available selective laser melting machines, suchas the EOS M280. In particular, it has been found that debris (e.g.condensate, powder particles etc) ejected during the lasersolidification (e.g. melting) process can be deposited on areas of thepowder layer that have yet to be solidified. This has been found toproduce additional surface roughness and the formation of layers ofnon-uniform thickness, which additionally create defects (e.g. pores,inclusions etc). Although altering the way in which the gas flows overthe powder bed has been proposed previously, the present inventors havefound that the stripe formation direction relative to the gas flowdirection has a significant effect on the quality of the layer that isformed. In particular, the present inventors have found that improvedpowder layer formation occurs if the predefined gas flow direction isalways at least partially opposed to the stripe formation direction. Inother words, the apparatus can be improved by ensuring there is always acomponent of the vector that describes the gas flow direction thatopposes the vector that defines the stripe formation direction.

It is important to note that the benefits of controlling the stripeformation direction relative to the gas flow direction had notpreviously been recognised by those skilled in the art. As explainedbelow, prior art techniques typically use stripe patterns in whichopposite stripe formation directions are used for adjacent stripes; thisis done in order to minimise the time taken to scan a laser beam overthe powder layer. For machines in which the gas flow is uncontrolled,the variation in gas flow direction relative to the stripe formationdirection is random and simply leads to a general reduction in materialuniformity across the powder layer. For machines in which gas flow isprovided along a certain predefined direction (e.g. as per the planarflow provided in the EOS M280 machine), the alternating direction ofstripe formation direction causes adjacent stripe of solidified materialto have different physical properties (e.g. different densities andsurface roughness). These variations are visible and produce weaknesseswithin three dimensional objects constructed from a plurality of suchlayers.

The present invention, by ensuring the predefined gas flow direction isalways at least partially opposed to the stripe formation direction,allows the quality and uniformity of components made by lasersolidification to be improved. Ensuring that stripe or stripe segmentformation does not occur in the presence of a “tail wind” means thatless ejected debris (condensate, powder particles etc) is deposited onpowder that is molten or is yet to be melted. Debris is thus carriedaway from the melt front (i.e. the part of the stripe that is presentlyin the molten state) and the accumulation of debris at the melt frontthat is otherwise seen is avoided. Preventing or reducing debrisaccumulation at the melt front, and deposition on top of regions of thepowder layer that have not yet been solidified, not only improves layerthickness and process uniformity but can also improve the overallefficiency of the solidification process. In particular, preventing sucha build up of debris ensures that there is no significant attenuation ofthe laser beam by debris before it reaches the powder layer to besolidified (thereby ensuring efficient solidification) and also preventspreviously deposited debris from being re-melted and ejected again fromthe surface.

The present invention is characterised by the stripe formation directionalways being at least partially opposed to the predefined gas flowdirection. In other words, there is always a component of the stripeformation direction vector that is in the opposite direction to the gasflow direction vector. The stripe formation direction may be completelyopposite (anti-parallel) to the gas flow direction or there may be anoblique angle (e.g. of less than 80°, more preferably of less than 70°or more preferably of less than 60°) between the stripe formationdirection and the gas flow direction. It should be noted that the signof such an oblique angle must still be selected to ensure that stripeformation direction is always at least partially opposed to thepredefined gas flow direction. Providing a stripe formation directionthat is completely opposite (anti-parallel) to the gas flow directionprovides optimum performance, but maintaining a single stripe formationdirection when constructing objects from multiple layers may not alwaysbe desirable, as described in more detail below. Conveniently, thestripe formation direction subtends an angle (α) of at least 10° to thenormal to the gas flow direction. More preferable, the angle (α) is atleast 15°, 20°, 25°, 30°, 35°, 40°, or 45. In a preferred embodiment,the stripe formation direction subtends an angle (α) of more than 30° tothe normal to the gas flow direction. This ensures that any hatch linesthat are used to form the stripes do not become parallel orsubstantially parallel to the direction of gas flow.

In addition to controlling the stripe formation direction relative tothe predefined gas flow direction, it has been found that the order inwhich the plurality of stripes or stripe segments are formed can affectthe uniformity and roughness (e.g. the density) of the solidified layer.If the flow of gas over the powder bed originates from a first side ofthe powder bed (e.g. if a gas outlet is provided at the first side ofthe powder bed), it is preferred that the plurality of stripes or stripesegments are formed in reverse order of their proximity to the firstside of the powder bed. Forming the stripes or stripe segments in suchan order ensures that debris (e.g. condensate or ejected particles)generated at the melt front is carried by the gas flow to areas of thepowder layer that have already been solidified, rather than beingdeposited on material that has yet to be solidified. This ensures thatsubsequent solidification is only of the fresh powder layer (i.e. apowder layer on which minimal debris has been deposited). This againhelps improve layer thickness uniformity and reduces surface roughness,thereby reducing defects.

The gas flow unit passes gas over the powder bed along a predefined gasflow direction. The gas flow may be along a linear gas flow direction orit may be along a non-linear (e.g. curved) gas flow direction. The gasflow direction may vary as a function of the position on the powder bed.Preferably, the gas flow direction is uniform over the powder bed (e.g.a planar gas layer may be produced). The gas flow unit may comprise atleast one gas outlet. The at least one gas outlet may comprise a linearbar with a plurality of spaced apart gas nozzles. The gas flow unit maycomprise at least one gas exhaust. The at least one gas exhaust maycomprise a linear bar with a plurality of exhaust vents. The gas flowunit may include a gas pump. The at least one gas outlet and the atleast one gas exhaust are preferably placed either side of the powderbed such that gas pumped from the at least one gas outlet passes to theat least one gas exhaust. Preferably, a substantially planar flow of gasis generated along the predefined gas direction. Preferably, asubstantially laminar flow of gas is generated along the predefined gasdirection. This planar or laminar flow of gas may be generated bypassing gas from the at least one gas outlet passes to the at least onegas exhaust.

Advantageously, the gas flow unit generates a gas flow that moves with avelocity of at least 2 m/s over the powder bed. The gas flow rate ispreferably selected so as to not disturb any deposited powder layer, butto allow ejected debris to be blown clear. Any gas, e.g. air, may bepassed over the powder bed. Advantageously, an inert gas is passed overthe powder bed. The inert gas may be at least one of nitrogen, argon andhelium.

The required pattern may be formed from a plurality of stripes. Thestripes may be elongate stripes. The plurality of stripes may comprisestripes of any shape. For example, a plurality of curved stripes may beformed. Preferably, each of the plurality of stripes comprises a linearstripe having an elongate axis. The stripe formation direction is thenpreferably parallel to the elongate axis of the stripe. The plurality ofstripes may be formed in any order, although as described above it ispreferred to form stripes in reverse order of their proximity to thesource of the gas flow. The laser beam may be advanced along an entirestripe before moving on to the next stripe. In other words, selectivemelting of one stripe may be completed before starting to melt adifferent stripe. Alternatively, only a part or section of one stripemay be formed before moving onto a part or section of another stripe. Itis, of course, possible for only part of a stripe to be solidified inorder to define the part of the pattern that is contained within thatstripe. The apparatus may include a controller for controlling the laserscanning unit that defines the plurality of stripes that are to be used.Each stripe may have a width of at least 5 mm, at least 10 mm, at least15 mm or at least 20 mm. Each stripe may have a width of no more than 50mm, 40 mm, 30 mm or 20 mm.

The required pattern may alternatively be formed from a plurality ofstripe segments. For example, the required pattern may be formed from aregular grid of stripe segments that form a checkerboard pattern. Thecheckerboard pattern may comprise a plurality of square or rectangularstripe segments. The stripe segments may all be the same size or may bedifferent sizes. An irregular pattern of stripe segments (e.g. islands)may also be formed. The formation of such stripe segments or sections isdescribed in, for example, US2005/0142024. All the stripe segmentswithin a layer may be formed by advancing the laser beam along the samestripe formation direction. Alternatively, a plurality of differentstripe formation directions may be used for different stripe segments ofa powder layer. For example, the different stripe segments may be formedusing different stripe formation directions. As explained above, thepresent invention ensures the stripe formation direction is always atleast partially opposed to the gas flow direction when writing eachstripe segment.

The laser scanning unit may comprise a pulsed laser. Preferably, thelaser scanning unit comprises a continuous wave (CW) laser. The laserscanning unit may include a laser beam modulator. The modulator may thenmodulate (e.g. activate or deactivate) the laser beam that impinges onthe powder layer. In this manner, laser solidification can be controlledby turning on and off the laser beam as required. Alternatively, thelaser power may be modulated, e.g. a sine wave modulation may beapplied. The laser scanning unit may generate a laser beam of sufficientpower to sinter and/or melt a powder layer. The power of the laser beammay be adjustable by a user.

The laser scanning unit may generate a laser beam that is appropriatelyshaped (e.g. by beam shaping optics) to form a variable length laserline having a long axis that extends across the stripe or stripesegment. In such an example, the pattern within the stripe is formed byadvancing the laser line along the stripe or stripe segment in thestripe formation direction. Advantageously, the laser scanning unitgenerates a laser spot that is moved (e.g. stepped or scanned) acrossthe stripe to form a so-called hatch line. The laser spot may have asubstantially circular cross-sectional profile (e.g. a Gaussian beamprofile). The laser scanning unit may form the hatch line by rapidlymoving (e.g. stepping or scanning) the spot from one side of the stripeto the other. This may be done using appropriate beam steering optics ofthe laser scanning unit. Preferably, each of the plurality of stripes orstripe segments are formed using a plurality of hatch lines that areadvanced along the stripe formation direction. In other words, hatchlines across the stripe or stripe segment are formed in succession witheach hatch line being located further along the stripe formationdirection than the preceding hatch line. It should, of course, be notedthat not all of the powder layer within a stripe or stripe segment mayneed to be melted to form the desired pattern. The formation of partialhatch lines, or the omission of hatch lines along selected parts of astripe or stripe segment, would be possible by appropriate control ofthe laser scanning unit.

The apparatus may form the series of hatch lines by raster scanning thelaser beam back and forth across the stripe or stripe segment. Theseries of hatch lines may also be formed by rapidly stepping the laserbeam back and forth across the stripe or stripe segment. The formationof such bidirectional hatch lines (i.e. hatch lines formed by lasermotion across the width of the stripe in two, opposite, directions) isthe conventional hatch line formation technique used to form therequired pattern along a stripe. As is also known, the laser beam may bedeactivated when the beam steering optics are being used to repositionthe laser beam from one hatch line to another.

The apparatus of the present invention may form the plurality of stripesor stripe segment using bidirectional hatch lines. Alternatively, aseries of hatch lines may be formed by only moving the laser beam in thesame line direction across the stripe or stripe segment. In other words,unidirectional hatch lines may be formed. Preferably, the line directionacross the stripe or stripe segment is at least partially opposed to thegas flow direction. In this manner, debris ejected during hatch lineformation is carried away from the direction in which the laser beam isadvancing across the stripe or stripe segment. Although the improvementassociated with using unidirectional hatch lines is relatively smallcompared to the benefits of aligning the stripe formation direction tothe gas flow direction, it does still provide a useful improvement. Thisis, however, at the expense of significantly reducing the speed at whichthe hatch lines can be written.

In one embodiment, the way in which the hatch lines are formed may bevaried during use. In particular, the way in which the hatch lines areformed may be selected depending on the orientation of the stripeformation direction relative to the gas flow direction. In other words,the line direction may be selected based on the orientation of thestripe formation direction relative to the gas flow direction. If theangle (α) between the stripe formation direction and the normal to thegas flow direction is substantial (e.g. greater than 10° , 20° or 30°)then bidirectional hatch lines may be used without any substantialdegradation in layer formation. However, for smaller angles (e.g. α lessthan 10° , 20° or 30°) it may be advantageous to use unidirectionalhatch lines. In such a case, the line direction can be selected to atleast partially oppose to the gas flow direction.

Apparatus of the present invention is typically used to build,layer-by-layer, three dimensional objects. The apparatus is thuspreferably arranged to deposit and selectively solidify a plurality ofpowder layers, each layer deposited on top of the preceding layer, toform a three dimensional object. It is preferred that the layerformation technique of the present invention is applied to each layer,in turn, of the object that is formed. In particular, the requiredpattern of each powder layer may be formed from a plurality of stripesor stripe segments. The stripe formation direction is preferably alwaysat least partially opposed to the predefined gas flow direction for eachstripe or stripe segment of each layer. In other words, it is preferredthat no stripes or stripe segments are formed that do not meet therequirement that the stripe formation direction is always at leastpartially opposed to the predefined gas flow direction. This ensuresuniformity and tight layer thickness control throughout the threedimensional object.

It should be noted that it is possible to perform a contour scan beforeand/or after the required pattern has been written into a powder layerusing the stripe or stripe segment based formation technique. A contourscan involves rapidly scanning the laser beam around the contour of thepart to re-melt/re-solidify material that will form the outer surface ofthe part being formed. Such a contour scan involves the solidificationof only a small amount of material thereby ejecting minimal amounts ofdebris and can thus be formed in a conventional manner (i.e. with nocontrol over laser beam movement direction relative to the gas flowdirection). Such a contour scan may be performed between the stripeformation technique being applied to a plurality of different powderlayers.

The apparatus of the present invention preferably also includes powderdeposition apparatus for depositing a powder layer onto the powder bed.The powder deposition apparatus preferably comprises a powder dispenserand a powder wiper. The powder bed may also include a moveable platformor table. A base plate may be attached to platform. The height of theplatform within the machine may be adjustable to allow the powder bed tobe dropped prior to the deposition of a powder layer. Such features areconventional for selective solidification machines and will not bedescribed further herein for brevity.

It has been explained previously in U.S. Pat. No. 8,034,279 thataltering the stripe direction for different layers is advantageous whenforming a three dimensional object. This also applies when using thetechnique of the present invention. In particular, different stripeformation directions (e.g. differing by 30°) are preferably used foradjacent powder layers. As explained above, it is preferred that thestripe formation direction used for each layer is at least partiallyopposed to the gas flow direction.

Apparatus of the present invention may be used to selectively sinterpowder. Advantageously, the apparatus of the present invention is usedto selectively melt powder. The powder may, for example, compriseplastic powder, ceramic powder or metal powder. Preferably, theapparatus is arranged to deposit and solidify metal powder. For example,metal powders such as steel (e.g. steel grade 1.2709), Stainless steel,titanium, cobalt chrome etc may be used.

According to a second aspect, the present invention provides a method ofselectively solidifying a powder layer deposited on a powder bed,comprising the steps of; passing a flow of gas over the powder bed alonga predefined gas flow direction, and scanning a laser beam over thepowder layer to selectively solidify at least part of the powder layerto form a required pattern, the required pattern being formed from aplurality of stripes or stripe segments, each stripe or stripe segmentbeing formed by advancing the laser beam along the stripe or stripesegment in a stripe formation direction, characterised by the stripeformation direction always being at least partially opposed to thepredefined gas flow direction. The method may include any of thepreferred features described for the corresponding apparatus of thefirst aspect of the present invention.

According to a further aspect, the invention provides selective lasermelting apparatus, comprising; a powder bed onto which a layer of powdercan be deposited, a gas flow device for passing a flow of gas over thepowder bed along a predefined gas flow direction, and a laser scanningunit for scanning a laser beam along a scan path on a layer of powderdeposited on the powder bed, the laser beam being selectively output asthe scan path is traversed to allow selected regions of the layer ofpowder to be melted, wherein the direction along which the laser beam ismoved is oriented relative to the gas flow direction so as tosubstantially prevent any particles ejected during laser melting frombeing carried by the gas flow into regions of the scan path that haveyet to be scanned.

In a further aspect, the present invention extends to selective lasersolidification apparatus, comprising; a powder bed onto which a powderlayer can be deposited, a gas flow unit for passing a flow of gas overthe powder bed along a predefined gas flow direction, and a laserscanning unit for scanning a laser beam over the powder layer toselectively solidify at least part of the powder layer to form arequired pattern, the required pattern being formed from a plurality ofstripes or stripe segments, wherein the flow of gas over the powder bedoriginates from a first side of the powder bed and the plurality ofstripes or stripe segments are formed in reverse order of theirproximity to the first side of the powder bed. The invention alsoextends to a corresponding method of operating selective lasersolidification apparatus and may include any feature described herein.

In a further aspect, the present invention extends to selective lasersolidification apparatus, comprising; a powder bed onto which a powderlayer can be deposited, a gas flow unit for passing a flow of gas overthe powder bed along a predefined gas flow direction, and a laserscanning unit for scanning a laser beam over the powder layer toselectively solidify at least part of the powder layer to form arequired pattern, the required pattern being formed from a plurality ofstripes or stripe segments, wherein the laser scanning unit generates alaser spot that is moved across the stripe or stripe segment to form ahatch line, each of the plurality of stripes or stripe segments beingformed using a plurality of hatch lines that are advanced along thestripe formation direction, characterised in that all hatch lines areformed by moving the laser beam in the same line direction across thestripe or stripe segment, the line direction being at least partiallyopposed to the gas flow direction. The invention also extends to acorresponding method of operating selective laser solidificationapparatus and may include any feature described herein.

In a further aspect, the present invention extends to selective lasersolidification apparatus, comprising; a powder bed onto which a powderlayer can be deposited, a gas flow unit for passing a flow of gas overthe powder bed along a gas flow direction and, a laser scanning unit formoving a laser beam over the powder layer to selectively solidify atleast part of the powder layer to form a required pattern, wherein thelaser scanning unit moves the laser beam to form a series of hatch linesthat are advanced over the powder layer along a hatch line movementdirection, wherein the hatch line movement direction is at leastpartially opposed to the gas flow direction. The hatch lines may be useto form stripes, segments, shells or any shape. The hatch line movementdirection may be different for different areas on the powder bed. Theinvention also extends to a corresponding method of operating selectivelaser solidification apparatus and may include any feature describedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a schematic illustration of a selective laser melting machine,

FIG. 2 shows a prior art technique for solidifying a layer of powderusing hatch lines advanced along twelve stripes,

FIG. 3 is a photomicrograph that shows the variation in oxidisation thatoccurs using the prior art technique of FIG. 2 ,

FIG. 4 illustrates the deposition of ejected material that occurs whenthe gas flow is in the same direction as the stripe formation direction(the “tail-wind” situation),

FIG. 5 illustrates the deposition of ejected material that occurs whenthe gas flow direction opposes the stripe formation direction as per thepresent invention,

FIG. 6 shows a first example of a stripe scan of the present invention,

FIG. 7 shows a second example of a stripe scan of the present invention,

FIGS. 8A, 8B and 8C show three options for forming hatch lines by rapidscanning of a laser beam across the width a stripe,

FIG. 9 illustrates how the stripe formation direction can be variedbetween layers, and

FIG. 10 illustrates how multiple stripe segments may be formed within alayer in a checkerboard pattern.

DETAILED DESCRIPTION

Referring to FIG. 1 , a known selective laser melting machine 2 isschematically illustrated.

The laser melting machine 2 comprises a build chamber or housing 4 inwhich there is provided a powder bed 6. The powder bed 6 can be raisedand lowed (i.e. moved in the z-direction) by a piston mechanism 8. Apowder dispensing and roller system 10 is provided for depositing a thin(e.g. 10-100 m) powder layer onto the top of the powder bed 6. Thepowder used to form the powder layer is preferably a metal powder (e.g.1.2709 grade steel powder).

A laser scanning unit 20 is also provided that comprises a high powercontinuous wave (CW) laser and scanning optics to direct a laser beam 22towards the powder bed 6. The scanning optics also allow the laser beam22 to be moved rapidly over the surface of the powder bed 6. The laserscanning unit 20 also includes an optical modulator to enable the laserbeam 22 that impinges on the powder layer to be turned on and off asrequired.

A gas flow unit 30 is also provided. The gas flow unit 30 comprises agas outlet bar 32 having a plurality of nozzles 34 for ejecting gas. Agas exhaust bar 36 is also provided for collecting gas. A pump 38 isused to draw in gas from the gas exhaust bar 36 and to pump gas to thenozzles 34 of the gas outlet bar 32. Suitable gas tubing 40 is providedto connect the gas outlet bar 32 and gas exhaust bar 36 to the pump 38.In use, gas flows from the gas outlet bar 32 to the gas exhaust bar 36.There is thus a predefined gas flow within the machine; i.e. gas ispassed over the powder bed along the gas flow direction G.

The laser melting machine 2 is operated, under the direction of acontroller 50, as follows. Firstly, a substrate plate is affixed to thepiston mechanism. The substrate plate, which is preferably formed fromthe same material as the powder to be deposited on it, forms the base ofthe powder bed. The powder dispensing and roller system 10 is then usedto dispense a powder layer of a certain thickness (e.g. 80 μm) onto thesubstrate plate. The laser scanning unit 20 then directs the laser beam22 onto the powder layer and melts selected parts of the powder layer;i.e. selected regions of the powder layer are melted to the substrateplate. The path over the powder bed that is used by the laser is scannedis described in more detail below. Once the required pattern (e.g.cross-section) has been written into the powder layer, the piston 8drops the powder bed 6, another powder layer is deposited on top of theexisting (partly solidified) layer and the laser scanning unit thenselectively melts the newly deposited powder layer. This process is thenrepeated, layer-by-layer, to form the required three dimensional object.During this fabrication process, a continuous supply of gas is passedover the powder bed along the gas flow direction G by the gas flow unit30.

The illustration and description of FIG. 1 shows only the basicoperation of known laser melting machines. The skilled person would beaware of further details of the machine construction and operation. Itshould be noted that the above schematic illustration is based on theM280 model of laser melting machine that is made by EOS GmbH.

Referring next to FIG. 2 , a prior art process for selectively melting adeposited powder layer using the machine described above with referenceto FIG. 1 will be described. This process is implemented as standard onthe EOS M280 machine mentioned above.

FIG. 2 illustrates a powder layer 100 that is to be selectively meltedto form the solidified layer pattern 102. The powder layer 100 isdeposited on the powder bed using the powder dispensing and rollersystem 10 that is described with reference to FIG. 1 . Also shown is thegas outlet bar 32 and the gas exhaust bar 36 that provide a planar flowof gas along a gas flow direction G.

In order to solidify the powder layer 100 to form the solidified layerpattern 102, a plurality of stripes (labelled S1-S12 in FIG. 2 ) aredefined. The stripes S1-S12 together define a square region thatcontains the area on the powder bed where the solidified layer pattern102 is to be written. The laser scanning unit 20 generates a laser spotthat is rapidly scanned across the width of the stripe (i.e. along adirection perpendicular to the elongate axis or length of the stripe) toform a so-called hatch line. In order to selectively melt powder alongthe length of the stripe, successive hatch lines are moved along thestripe in the direction L. In other words, the stripe is formed bymovement of the hatch line along the stripe formation direction L. Itshould be noted that the hatch line formed by the laser may be the widthof the stripe or it may be shorter than the width of the stripe ifmelting is not required across the whole stripe width at that particularposition.

In the prior art example shown in FIG. 2 , the stripe S12 is addressedfirst. This involves the laser scanning unit forming a hatch line thatis moved from left to right along the stripe formation direction L tosolidify the part of the layer pattern 102 falling with the S12 stripe.After stripe S12 has been written, the pattern of stripe S11 is written,which involves moving the hatch line along the stripe S11 in the stripeformation direction L. The stripe formation direction L for stripe S11is opposite to the stripe formation direction L for stripe S12. It canthus be seen that the stripe formation direction L alternates betweenstripes as the stripes are written in turn (i.e. in the order S12 toS1). In the present example, all even numbered stripes (S2, S4, S4 etc)are formed using a hatch line that is moved from left to right whereasall odd numbered stripes (S1, S3, S5 etc) are formed using a hatch linethat is moved from right to left.

The present inventors have found that this prior art technique has anumber of disadvantages. As shown in FIG. 3 , it has been found by thepresent inventors that stripes formed by hatch line movement in opposedstripe formation directions have different surface discoloration andsurface roughness. In particular, the present inventors have found thatoxidization and high surface roughness occurs for even numbered stripes(S2, S4, S4 etc) that have been formed using a hatch line that is moved(in FIG. 2 ) from left to right. These even numbered stripes areidentified by the label 140 in the photomicrograph of FIG. 3 .

Referring now to FIGS. 4 and 5 , the effect of the gas flow directionrelative to the stripe formation direction on layer formation will bedescribed.

FIG. 4 shows a powder bed 200 that carries a layer of melted metal 202and a powder layer 204 that has yet to be melted. The dashed lines 206illustrate the hatch lines that were generated by the laser scanningunit to melt the powder that now forms the layer of melted metal 202. Inthis example, the gas flow direction G is the same as the stripeformation direction L. In other words, there a component of the gas flowdirection that is in the same direction as the stripe formationdirection L; this could be thought of as there being a “tail-wind”.

The inventors have found that when the gas flow direction G and stripeformation direction L are aligned in the manner shown in FIG. 4 , debrisfrom the melting process (powder particles, partially melted clumps ofpowder particles and other residue/condensate from the melting processetc) are carried by the gas flow towards the part of the powder layerthat has yet to be melted. This debris forms a surface region or bulgeof contaminant 208 that moves along the stripe as the melt progresses.This not only results in a layer of non-uniform thickness being formedbecause of the different sized particles being deposited on the top ofthe unmelted powder, but it also reduces the laser power that reachesthe powder layer thereby altering the melting conditions of theunderlying powder layer. In particular, it has been found thatsub-optimum oxidisation of the melted powder occurs and that the processgenerates a relatively high level of surface roughness and introducesdefects etc. The effect shown in FIG. 4 accounts for the poorer qualityof the even numbered stripes (S2, S4, S4 etc) shown in thephotomicrograph of FIG. 3 .

FIG. 5 shows a powder bed 300 that carries a layer of melted metal 302and a powder layer 304 that has yet to be melted. The dashed lines 306illustrate the hatch lines that were generated by the laser scanningunit to melt the powder that now forms the layer of melted metal 302. Inthis example, the gas flow direction G is opposed to the stripeformation direction L. In other words, there is no component of the gasflow direction that is in the same direction as the stripe formationdirection L; i.e. there is no “tail-wind”.

In this example, the flow of gas in the gas flow direction G acts toblow debris from the melting process away from the powder layer of thestripe that has yet to be melted. This has been found to prevent theformation of a bulge of contaminant as illustrated in FIG. 4 . It shouldbe noted that although FIG. 5 illustrates the use of gas flow directionG that is fully opposed to the stripe formation direction L, there mayinstead be an oblique angle between the gas flow direction G and thestripe formation direction L. Providing such an oblique angle alsoensures that the debris is not deposited onto regions of the stripe thathave just been melted.

It should also be noted FIGS. 4 and 5 illustrate embodiments in whichthe whole width of the stripe is melted by a plurality of full widthhatch lines that are formed at successive points along the stripeformation direction L. It is, of course, possible to only melt selectedparts of each stripe in order to construct the desired cross-section orpattern of melted material.

FIG. 6 illustrates the powder layer 400 that is to be selectively meltedto form the solidified layer pattern 402. The powder layer 400 isdeposited on the powder bed using the powder dispensing and rollersystem that is described with reference to FIG. 1 . Also shown are thegas outlet bar 32 and gas exhaust bar 36 described with reference toFIG. 1 that provide a flow of gas along a gas flow direction G.

In order to solidify the powder layer 400 to form the solidified layerpattern 402, a plurality of stripes are melted in turn; these stripesare labelled as S1-S12 in FIG. 6 . Unlike the prior art processdescribed above with reference to FIG. 2 , the stripes illustrated inFIG. 6 formed by moving the hatch line along each stripe in 5 the samedirection. In other words, the same stripe formation direction L is usedfor each of the stripes S1 to S12. In addition, the stripe formationdirection L is arranged to differ from the gas flow direction G by theangle θ, which in this example is about 125°. Also shown in FIG. 6 isthe angle α between the normal to the gas flow direction G and thestripe formation direction L. In this example, α has a value of around35°.

Providing such an angle between the gas flow direction G and the stripeformation direction L means that any debris ejected during the meltingprocess is carried by the flow of gas away from the part of the powderlayer that is yet to be melted and also away from any material of thatstripe that has just been melted. For example, debris ejected from thesurface when melting the point P shown in FIG. 6 is carried along thevector d and away from the stripe S1. This helps ensure that themajority of the debris does not cover powder yet to be sintered and alsodoes not adhere to powder that has recently been melted. The solidifiedlayer pattern 402 formed by the melting process thus has a more uniform(less rough, fewer defects) surface than layers produced usingalternating stripe formation directions as per the prior art processdescribed with reference to FIG. 2 .

In addition to each stripe being formed by moving a hatch line along thesame stripe formation direction L, the stripes S1 to S12 are preferablyformed in a specific order. In particular, the stripes S1 to S12 arepreferably formed in reverse order of their proximity to the gas outletbar 32. In other words, the stripe Si nearest the gas exhaust bar 36 isformed first, then stripe S2 is formed, then stripe S3 etc. Forming thestripes in this order has the advantage that any debris ejected whenwriting one stripe does not disrupt the powder layer for stripes thathave yet to be written. In particular, it can be seen that any debrisejected whilst melting the selected parts of the powder layer withinstripe S1 does not get carried in the flow of gas over the stripesS2-S12. This means that a more uniform, substantially debris free,powder layer is present when each stripe is written.

Referring to FIG. 7 , a powder layer 500 is shown that is to beselectively melted to form the solidified layer pattern 502. The powderlayer 500 is deposited on the powder bed using the powder dispensing androller system that is described with reference to FIG. 1 . Also shownare the gas outlet bar 32 and gas exhaust bar 36 described withreference to FIG. 1 that provide a flow of gas along a gas flowdirection G.

In this example, the stripes S1 to S12 are again preferably formed inreverse order of their proximity to the gas outlet bar 32. Each stripeis formed by moving the hatch line along each stripe in the samedirection. In other words, the same stripe formation direction L is usedfor each of the stripes S1 to S12. It can also be seen that the stripeformation direction L of FIG. 7 is a reflection of the stripe formationdirection L shown in FIG. 6 about the gas flow direction. In otherwords, the stripe formation direction L is arranged to differ from thegas flow direction G by the angle −θ in FIG. 7 . The arrangement of 7thus has similar benefits to that of FIG. 6 .

In addition to optimising the stripe formation direction L, it should beremembered that each stripe is preferably formed using a series of hatchlines. These hatch lines a formed by scanning a laser spot across thestripe; i.e. the hatch line is formed by moving a laser spot along aline that is perpendicular to the stripe formation direction L. It hasfurther been found that a further improvement to the uniformity ofstripe formation can be obtained by altering the hatch line formationprocess. This will now be explained with reference to FIGS. 8A, 8B and8C.

Referring to FIG. 8A, there is shown a prior art method for scanning alaser spot back and forth across the width of a stripe S to form asuccession of hatch lines 600 along the stripe formation direction L.The dashed lines 602 illustrate the notional paths at the end of eachhatch line 600 that are traversed (with the laser beam deactivated) inorder to appropriately position the laser beam ready for formation ofthe next hatch line. For convenience, the series of hatch lines of FIG.8A can be termed bidirectional hatch lines.

The technique of hatch line formation shown in FIG. 8A has the advantagethat the successive hatch lines can be formed at high speed because thebeam steering optics of the laser scanning unit only need to provide asmall amount of (notional) laser beam movement between the end of onehatch line and the start of the next hatch line. It has been found,however, that the direction of hatch line formation relative to the gasflow direction can also affect the quality and uniformity of the layerthat is generated from melting the powder layer within a stripe. It hasalso been found that the non-uniformity caused by this effect increasesas the magnitude of the angle α (which is described above with referenceto FIGS. 6 and 7 ) between the normal to the gas flow direction G andthe stripe formation direction L reduces.

Forming the hatch lines by always scanning the laser beam in the samedirection across the stripe can thus improve the uniformity of themelted layer. FIG. 8B shows how hatch lines 610 can be formed by alwaysscanning the laser spot from the top to the bottom of a stripe S. FIG.8C shows how hatch lines 620 can be formed by always scanning the laserspot from the bottom to the top of a stripe S. For convenience, theseries of hatch lines of FIGS. 8B and 8C can be termed unidirectionalhatch lines. Although the formation of unidirectional hatch lines canimprove stripe quality, such an improvement is accompanied by anincrease in the time it takes to form a series of hatch lines. Inparticular, there is an additional delay associated with the scanningoptics of the laser scanning unit moving back across the stripe to allowthe next hatch line to be formed. It is thus preferable to only useunidirectional hatch lines when they provide a sufficiently significantimprovement; e.g. when the magnitude of the angle a between the normalto the gas flow direction G and the stripe formation direction L reducessufficiently so that the formation of unidirectional hatch lines has abenefit.

It should also be noted that the direction of formation of theunidirectional hatch lines relative to the stripe formation direction Lwill depend on the orientation of the stripe being formed relative tothe gas flow direction G. For example, the stripes formed in FIG. 6would benefit from being formed from the unidirectional hatch lines ofFIG. 8B whilst the stripes formed in FIG. 7 would benefit from beingformed from the unidirectional hatch lines of FIG. 8C. In both cases,the direction of beam movement during hatch line formation is arrangedto at least partly oppose the gas flow direction G. This means that themajority of debris associated with powder melting is blown clear ofpowder within the hatch line that is yet to be melted. The formation ofbidirectional or unidirectional hatch lines may thus be varied asrequired for different stripe orientations relative to the gas flowdirection.

Referring to FIG. 9 , the process of constructing a three dimensionalobject 710 from a plurality of melted layers (700 a-700 f) isillustrated. Each layer 700 a-700 f may be formed by a process thatinvolves selectively melting each layer using multiple stripes, eachstripe of one layer being formed along a common stripe formationdirection L. The stripe formation direction L may vary between layers,but it is preferred that the gas flow direction is always at leastpartially opposed to the stripe formation direction for each layer. Inthis manner, the benefits of the present invention are obtained for eachlayer in a three dimensional object. The use of different stripeformation directions L for each layer may also mean that certain layerscan be formed using bidirectional hatch lines whilst other layers areformed using unidirectional hatch lines. The benefits of altering thestripe formation direction between layers has also been describedpreviously in U.S. Pat. No. 8,034,279 (EOS). A difference in stripeformation direction between adjacent layers of at least 30° is used inthis example, but other different angles may be implemented. Forexample, the rotation angle between adjacent layers may be more than 40°or it may be 67°. The rotation angle between adjacent layers ispreferably less than 80°. Again, it should be noted that it is preferredthat each layer (or at least the majority of the layers) meets therequirement that the gas flow direction is always at least partiallyopposed to the stripe formation direction.

It should also be noted that although the melting process may take placeby advancing hatch lines along a stripe, there may be other processingsteps that do not requires such tight control over the stripe formationdirection relative to the gas flow direction. For example, the laserscanning unit may perform a contour scan before and/or after a layer hasbeen melted by advancing hatch lines along a stripe. The contour scanmay simply scan the laser beam around the contour of an object tore-melt and solidify the metal to improve surface quality. In such acontour scan the path of the laser beam spot may take on any orientationrelative to the gas flow direction. This has not been found to have adetrimental effect because the amount of debris generated by such acontour scan is minimal.

Referring to FIG. 10 , it is also illustrated how the present inventioncan be applied to layer formation using stripe segments. In particular,FIG. 10 illustrates a powder layer 800 that is to be melted to form adesired pattern (not shown). The powder layer 800 may be deposited usingthe powder dispensing and roller system described with reference to FIG.1 . As described previously, a planar flow of gas is provided over thepowder layer along the direction G.

The desired pattern is written to the powder layer 800 by dividing thelayer into a plurality of stripe segments 802. Each stripe segment 802is formed separately. The stripe segments may be written in any order.As also shown in FIG. 10 , the stripe formation direction L may bedifferent for the different stripe segments (although it may be thesame). However, the gas flow direction G is always at least partiallyopposed to the stripe formation direction L of each stripe segment.

The stripe segments shown in FIG. 10 are equally sized and regularlyarranged in a grid or checkerboard type pattern. It is, however,possible for the stripe segments or sections to be different shapesand/or sizes. The stripe segments may also be spaced irregularly. Forexample, stripe segments may be provided as shells or islands formed inlocal areas on the substrate. The stripe segments or sections may alsocomprise any of the arrangements described previously in US2005/0142024(but with the gas flow direction controlled relative to the stripeformation direction in accordance with the present invention).

The skilled person would also recognise the other variations andadditions to the technique of the present inventions that are possible.

1. Selective laser solidification apparatus, comprising; a powder bedonto which a powder layer can be deposited, a gas flow unit for passinga flow of gas over the powder bed along a predefined gas flow direction,and a laser scanning unit for scanning a laser beam over the powderlayer to selectively solidify at least part of the powder layer to forma required pattern, the required pattern being formed from a pluralityof stripes or stripe segments, each stripe or stripe segment beingformed by advancing the laser beam along the stripe or stripe segment ina stripe formation direction, wherein the stripe formation direction isalways at least partially opposed to the predefined gas flow direction.2. An apparatus according to claim 1, wherein the stripe formationdirection subtends an angle (α) of more than 30° to the normal to thegas flow direction.
 3. An apparatus according to claim 1, wherein theflow of gas over the powder bed originates from a first side of thepowder bed and the plurality of stripes are formed in reverse order oftheir proximity to the first side of the powder bed.
 4. An apparatusaccording to claim 1, wherein the gas flow unit comprises at least onegas outlet and at least one gas exhaust, the at least one gas outlet andthe at least one gas exhaust being placed either side of the powder bedsuch that gas pumped from the at least one gas outlet to the at leastone gas exhaust provides a substantially planar flow of gas along thepredefined gas direction.
 5. An apparatus according to claim 1, whereinthe required pattern is formed from a plurality of elongate stripes. 6.An apparatus according to claim 5, wherein each of the plurality ofelongate stripes comprises a linear stripe having an elongate axis andthe stripe formation direction is parallel to the elongate axis.
 7. Anapparatus according to claim 1, wherein the required pattern is formedfrom a plurality of stripe segments.
 8. An apparatus according to claim1, wherein the laser scanning unit generates a laser spot that is movedacross the stripe or stripe segment to form a hatch line, each of theplurality of stripes or stripe segments being formed using a pluralityof hatch lines that are advanced along the stripe formation direction.9. An apparatus according to claim 8, wherein the laser scanning unitforms the series of hatch lines by moving the laser beam back and forthacross the stripe or stripe segment.
 10. An apparatus according to claim8, wherein all hatch lines are formed by moving the laser beam in thesame line direction across the stripe, the line direction being at leastpartially opposed to the gas flow direction.
 11. An apparatus accordingto claim 10, wherein the line direction is selected based on theorientation of the stripe formation direction relative to the gas flowdirection.
 12. An apparatus according to claim 1, wherein the apparatusdeposits and selectively solidifies a plurality of powder layers to forma three dimensional object, the required pattern of each powder layerbeing formed from a plurality of stripes or stripe segments, wherein thestripe formation direction is always at least partially opposed to thepredefined gas flow direction for each stripe or stripe segment of eachlayer.
 13. An apparatus according to claim 12, wherein different stripeformation directions are used for adjacent powder layers.
 14. Anapparatus according to claim 1, wherein the laser unit is arranged toselectively melt the powder layer.
 15. A method of selectivelysolidifying a powder layer deposited on a powder bed, comprising thesteps of; passing a flow of gas over the powder bed along a predefinedgas flow direction, and scanning a laser beam over the powder layer toselectively solidify at least part of the powder layer to form arequired pattern, the required pattern being formed from a plurality ofstripes or stripe segments, each stripe or stripe segment being formedby advancing the laser beam along the stripe or stripe segment in astripe formation direction, wherein the stripe formation directionalways being at least partially opposed to the predefined gas flowdirection.
 16. Selective laser solidification apparatus, comprising; apowder bed onto which a powder layer can be deposited, a gas flow unitfor passing a flow of gas over the powder bed along a predefined gasflow direction, and a laser scanning unit for scanning a laser beam overthe powder layer to selectively solidify at least part of the powderlayer to form a required pattern, the required pattern being formed froma plurality of stripes or stripe segments, wherein the flow of gas overthe powder bed originates from a first side of the powder bed and theplurality of stripes or stripe segments are formed in reverse order oftheir proximity to the first side of the powder bed.